Eosinophils normally account for 1B3% of the total peripheral blood leukocytes. A marked accumulation of eosinophils, a condition known as eosinophilia, can occur in many disorders such as allergic diseases, parasitic infections and cancer (Rothenburg 1998). Eosinophilia is classified as having greater than 350 eosinophils per cubic millimeter of blood, and in severe cases levels may rise to over 5000 cells per cubic millimeter. As well as accumulating in the peripheral blood of the diseased individual, eosinophils may also selectively accumulate in any tissue in the body. Such eosinophilia can be harmful due to the pro-inflammatory effects of the eosinophils. In eosinophilic conditions such as asthma, there is frequently a correlation between the number of infiltrating eosinophils and severity of the disease.
Eosinophils accumulate at inflammatory sites where they can survive for prolonged periods, depending on the combination of cytokines produced in their immediate environment. Eosinophils contain many toxic inflammatory mediators that are stored in granules. Upon activation by one or more of a wide number of cytokines, eosinophils degranulate to release these toxins that include cationic proteins, such as major basic protein, eosinophil-derived neurotoxin and eosinophilic peroxidase. In addition, activated eosinophils also release chemoattractants, lipid mediators such as leukotrienes and a wide range of inflammatory cytokines. Many of these substances have significant cytotoxic effects on tissues, such as the respiratory epithelium in asthma (Rothenberg, 1998).
Chemokines are a group of homologous 8B10 kDa proteins (Luster, 1998) that are subdivided into families based on the relative positions of the conserved cysteine residues. Chemokines play an important role in mediating leukocyte extravasation from the blood into tissues as they provide the directional signals for the movement of leukocytes during normal development and homeostasis, and importantly, in inflammation. Although there are numerous chemotactic substances, such as leukotriene B4, interleukins and bacterial products, that are able to recruit eosinophils to tissues, only the chemokine, eotaxin-1 has been shown to recruit eosinophils specifically.
Human eotaxin is a member of the rapidly expanding group of β or CC (Cys-Cys) subfamily of chemokines. This group of molecules is characterised by the presence of 4 conserved cysteines, the first 2 of which are adjacent and share a sequence identity between 20 and 75%. Members of this family include eotaxin-2 (Forssmann et al, 1997; White et al, 1997), eotaxin-3 (Shinkai et al, 1999), monocyte chemoattractant protein (MCP)-1, MCP-2, MCP-3, MCP-4, MCP-5 (Van Coillie et al, 1999), macrophage inflammatory protein (MIP)-1, MIP-1β, TARC, LARC, I309 and RANTES.
Eotaxin-1 is an 8.4 kDa, 74 amino acid protein that was first detected in the bronchoalveolar lavage (BAL) fluid from allergen challenged sensitised guinea pigs (Griffiths-Johnson et al, 1993; Jose et al, 1994a). The molecule was first identified as a potent chemoattractant as it induced a substantial accumulation of eosinophils at its intradermal injection site. The guinea pig gene was the first cloned (Jose et al, 1994b, Rothenberg et al, 1995a), followed by mouse (Rothenberg et al, 1995b). The human eotaxin gene was subsequently identified (Kitaura et al 1996; Garcia-Zepeda et al 1996; Ponath et al, 1996) and the rat homologue has more recently been cloned (Williams et al, 1998). Human eotaxin has 61% identity with mouse and guinea pig eotaxin, and 62% identity with rat eotaxin. The human gene is located on chromosome 17 and comprises of three exons and two introns. The 5′ flanking region of the gene contains a number of consensus regulatory elements, including binding sites for AP-1, NFB, interferon gamma response element and the glucocorticoid receptor, suggesting that gene expression is regulated by cytokines as well as by glucocorticosteroids.
Eotaxin can be produced by a variety of normal cell types including epithelial cells, fibroblasts, endothelial cells, T-lymphocytes, monocytes and macrophages (Cook et al, 1998; Ponath et al, 1996a; Li et al, 1997). Although eosinophils are the main effector cells for eotaxin, eosinophils also synthesize eotaxin themselves and store it in intracellular granules (Nakajima et al, 1998). The release of eotaxin from eosinophils may contribute to the local accumulation of eosinophils in inflammatory conditions. Eotaxin expression can be induced from the-different cell types by many pro-inflammatory mediators, such as tumour necrosis factor-alpha, interferon and interleukin-1.
Eotaxin-2 has recently been cloned (Forssmann et al, 1997; White et al, 1997). It does not exhibit close sequence homology with eotaxin, as it shares only 39% amino acid identity. Like eotaxin, however, eotaxin-2 is a chemoattractant for eosinophils and basophils, albeit up to 10Bfold less potent. Eotaxin-3 has also been recently been identified (Shinkai et al, 1999) but its potency also appears to be 10Bfold less than that observed for eotaxin. Consequently, eotaxin-3 is chemotactic for eosinophils and basophils only at relatively high concentrations (Kitaura et al, 1999).
In general, there is substantial redundancy in the binding of chemokines to chemokine receptors. Typically, several different CC chemokines are able to bind a single chemokine receptor, and conversely, a single CC chemokine can bind to several different chemokine receptors. The chemokine receptor, CCR3, has many ligands including eotaxin, MCP-2, MCP-3, MCP-4, RANTES, eotaxin-2 and 3. Of these, eotaxin appears to be the most important. Many of the ligands, such as MCP-2, MCP-3 and RANTES, have a relatively low affinity for CCR3 and are therefore not particularly effective at inducing CCR3 mediated events. In contrast, eotaxin binds to the CC chemokine receptor 3 (CCR3) with relatively high affinity, Kd=0.52 nM (Ponath et al, 1996a). Furthermore, eotaxin is unusual among CC chemokines in that it only binds to CCR3 and not to any other chemokine receptor, that is, eotaxin is specific for CCR3.
Human CCR3 has been cloned (Combadiere et al, 1995; Daugherty et al, 1996) and is a 355 amino acid, 41 kDa, seven transmembrane domain protein. It contains four cysteines in its extracellular domain and eight serine/threonine residues in the cytoplasmic tail that are potential sites for G-protein mediated phosphorylation. CCR3 has no potential sites for N-linked glycosylation. The human receptor binds both mouse and human eotaxin with equal affinity (Daugherty et al, 1996). Mouse (Gao et al, 1996) and guinea pig (Sabroe et al, 1998) CCR3 have subsequently been cloned and share 69 and 67% amino acid identity with human CCR-3, respectively.
Human CCR-3 is principally expressed on eosinophils (Ponath et al, 1996b) and basophils (Uguccioni et al 1997; Yamada et al 1997). It is also found on TH2-type T cells (Sallusto et al, 1997), microglial cells in the central nervous system (He et al, 1997) and dendritic cells (Rubbert et al, 1998). Eotaxin is a chemoattractant and activator of CCR3 expressing cells. On binding CCR3 on eosinophils, eotaxin causes intracellular calcium mobilisation, initiation of intracellular actin polymerisation, upregulation of integrin expresssion and the induction of oxygen radical production (Tenscher et al, 1996; Elsner et al, 1996). CCR3 is expressed at particularly high levels on eosinophils with 40,000 (Daugherty et al, 1996) to 400,000 (Ponath et al, 1996b) receptors per cell. Many CCR3 ligands, such as MCP-2, MCP-3, MCP-4 and RANTES, also bind chemokine receptors other than CCR3 and can therefore mediate chemoattraction of a wide variety of cell types. In contrast, due to its high selectivity for CCR3, eotaxin is able to specifically chemoattract and activate CCR3 expressing cells such as eosinophils.
There is a growing body of evidence that blocking the effects of eotaxin may used therapeutically. There are several in vivo studies that have used either rabbit or rodent antibodies. One such study looked at the effects of an intraveneously (iv) administered anti-eotaxin antibody. Gonzalo et al (1996) injected 20 μg an anti-eotaxin rabbit polyclonal antiserum iv into ovalbumin-challenged mice. Antibody administration prior to challenge reduced the eosinophilia by 56%, as measured by the number of eosinophils accumulating in broncho-alveolar lavage (BAL) fluid.
There are also a number of reports of the effects of locally administered anti-eotaxin antibodies. Humbles et al (1997) described the co-injection of guinea pig eotaxin (10 ng) with a rabbit polyclonal anti-eotaxin antiserum (10 μl) into the skin of naive guinea pigs that had received a prior injection of 111In-labelled eosinophils. The polyclonal antibody was able to completely block local eosinophil accumulation. Similarly, Teixeira et al (1997) used a mouse model of eosinophilia, in which murine eotaxin (1-30 pmol) was co-injected with a rabbit polyclonal anti-eotaxin antiserum intradermally into the sites of 4-8 hour active cutaneous anaphylactic reactions. Dilutions of 5% and 20% of the antiserum blocked eosinophil recruitment by 45% and 95%, respectively. In addition, Sanz et al (1998) have looked at eosinophil accumulation due to endogenously generated eotaxin induced by intradermal IL-4 injection. An anti-eotaxin polyclonal antiserum gave a 54% inhibition of the late phase (24B28 hr) but not the early phase (0B4 hr) of the response to IL-4.
To further understand the role of eotaxin in the healthy and eosinophil-mediated disease state, targeted gene disruption has been used to generate mice that are deficient in eotaxin (Rothenberg et al 1997). When these mice are sensitised and challenged with ovalbumin, eosinophil numbers were reduced by 70% in BAL from lungs of eotaxin null mice compared with wild type mice (18 hrs after challenge). This demonstrates that eotaxin enhances the magnitude of the eosinophil recruitment after antigen challenge in models of asthma. Nakamura et al. (Am. J. Resp. & Crit. Care Med. (1999) 160: 1952-1956) demonstrates association of eotaxin levels with asthma and inverse relation with lung function.
Eotaxin mRNA is constitutively produced by a number of tissues, where it has been suggested to play a role in eosinophil homing (Rothenberg et al 1995). In the eotaxin null mice, no gross histological abnormalities could be detected in any organ, including those known to express eotaxin. Similarly no changes in leukocyte phenotype could be detected. However, the total eosinophil count was reduced by 3-fold in the null mice compared to the wild-type, suggesting that eotaxin also plays a role in determining the baseline number of eosinophils in the peripheral circulation (Rothenberg et al 1997).
Specific binding members according to the present invention are useful in binding to and preferably neutralising eotaxin, with therapeutic potential in various diseases and disorders in which eotaxin plays a role. Exemplary diseases and disorders are discussed further below.